1. Field of the Invention
The present invention relates generally to an image forming apparatus, and more particularly, to a process speed control device used in an image forming apparatus of the type which forms an image by transferring a dye image onto an image receiving sheet of ordinary paper or OHP (film for overhead projector) coated with a thermoplastic resin, and heating the receiving sheet so as to melt the thermoplastic resin coat thereon until the molten coat covers the receiving sheet so as to enable the dye image thereon to be glossy.
2. Description of the Prior Art
An image forming apparatus is known which forms an image by superimposing a photosensitive sheet containing microcapsules encapsulating light-curable agents and a coloring dye on an image receiving sheet coated with a thermoplastic resin and a developer for developing the dye. When the photosensitive sheet is exposed to light, such as light reflected from a document, the microcapsules are selectively cured by the light, thus forming a latent image. The image receiving sheet is then superposed on the latent image portions and is pressed against them. Under the pressure uncured capsules are caused to rupture, thereby enabling the dyes inside to flow out over the image receiving sheet. The dye reacts with the developer, and forms a dye image on the image receiving sheet. Then, by heating the image receiving sheet on which the dye image is formed, the thermoplastic resin applied to the surface of the image receiving sheet is melted until a molten dye covers the surface of the image receiving sheet, thereby enabling the image receiving sheet to be glossy.
Irrespective of whether the backing of the image receiving sheet may be ordinary paper or OHP film, the process is conducted at the same speed. However, because of the different thermal capacity between ordinary paper and the OHP film, the OHP film is liable to insufficient heating when the receiving sheets using ordinary paper and OHP film are heated while they are being conveyed at the same speed, thereby resulting in insufficient softening or melting of the thermoplastic resin. This causes fatal white spots on the surface of the image receiving sheet.